The Sun: Energy from Nuclei

Introduction

Life on Earth is dependent on energy from our Sun. How the Sun produces this energy was a key question that challenged many scientists over the years. It emits about 3.8 × 1026 joules every second. Calculations easily showed that this could not be due to combustion or some normal chemical reaction - it would have used up its fuel long ago. Others mechanisms were suggested in the Nineteenth Century.

One model suggested that that meteors falling onto the Sun convert their kinetic energy into heat upon collision. This suggestion was discounted due to the lack of meteors observed and that for there to be sufficient meteors there would be many more collisions on Earth. Helmholtz and Lord Kelvin proposed in the 1850s that the energy arose due to the gravitational contraction of the Sun. In this model the Sun contracted by one part in 10,000 every 2000 years, giving it a lifespan of about 107 years. Stratigraphic analysis of rock layers by geologists however suggested that the Earth must be older than 100 million years. Subsequent calculations by Ernest Rutherford based on abundances of uranium isotopes in the early 1900s pushed this to at least 700 million years old. The discrepancy between the age of the Sun and that of the Earth would not be solved until the 1930s when the concept of nuclear fusion became accepted.

Nuclear Radiation

Radioactivity was discovered by Henri Becquerel in 1896, spurred by Wilhelm Röntgen's discovery of X-rays the previous year. Marie Sklodowska Curie and her husband, Pierre isolated a new radioactive element that they called radium from uranium ores. Rutherford and Frederick Soddy studied the radioactive emissions and suggested that there were two different types. They named these alpha, α and beta, β radiation. A third type, called gamma, γ was subsequently also found often associated with the emission of the other two types.

Types of Radioactive Emissions:

Alpha (α) Particles:

In alpha emission a particle with the composition of a helium nucleus, that is two protons and two neutrons or an alpha particle , is emitted from an unstable heavy nucleus. This results in the parent nucleus changing or transmutating into a daughter nucleus of the element with atomic number two lower than the parent. Alpha sources are usually found among the heavier radioactive isotopes including some of uranium and radium. The emitted alpha particle has a mass number of 4 and a charge of 2+ so is relatively heavy and highly ionising. This means that an alpha particle can do a lot of damage to cells in biological tissue that it hits but also travels only a short distance through most substances (including air) as it loses kinetic energy through multiple collisions with other particles.

A uranium-238 nucleus is unstable. It will spontaneously decay into a Thorium-234 nucleus, releasing an alpha particle (He-4 nucleus). The combined mass of the alpha particle and thorium nucleus is slightly less than that of the original uranium nucleus. The difference accounts for the energy released. As Einstein showed, mass can be converted into energy (E = m.c2).

Beta (β) Particles

Beta decay occurs when a neutron either within a radioisotope or as a free neutron spontaneously decays to form a proton, an electron and an electron antineutrino. The emitted electron has high kinetic energy and is called a beta (β) particle. Beta decay is a weak nuclear interaction with the W- particle acting as the force carrier (intermediate vector boson).

A neutron undergoes spontaneous beta decay to form a proton, a high energy electron (β particle) and an electron antineutrino. Whilst atomic and mass numbers are conserved in the process the combined mass of the products is slightly less than the original neutron mass. This accounts for the energy released.

Many radioisotopes undergo beta decay. The example below shows the beta decay of a Thorium-234 nucleus (which is initially produced by alpha decay of U-238 as shown above).

Beta decay of Thorium-234 results in a high energy electron (beta particle) being released from the nucleus plus an electron antineutrino (not shown in diagram). The daughter nucleus thus formed has the same mass number of 234 as the parent but is one higher in atomic number as the decaying neutron has changed into a proton which is retained within the nucleus. The Protactinium-234 nucleus still contains excess energy which it gets rid of almost instantaneously by emitting a high energy gamma photon, a gamma ray.

Beta particles have a charge of -1 and a mass number of 0 though they still have an actual rest mass about 1/1850th that of a proton.

Gamma (γ) Rays

Both alpha and beta decay events often produce daughter nuclei that still possess too much energy and are intrinsically unstable. These nuclei may spontaneously and almost instantaneously emit a high energy photon in the gamma radiation region of the electromagnetic spectrum. The photons are generally termed gamma rays (γ). Emission of a gamma photon does not alter the mass or atomic number of the daughter nucleus but does lower its energy.

Properties of Nuclear Radiation

All three types of nuclear radiation; alpha, beta and gamma, are highly energetic and can interact to varying degrees with matter including biological tissue. In general the greater the charge and mass of the type of radiation, the more damage it does when it hits or collides but this also means it travels less distance through air or other matter before giving up its energy.

Alpha particles are thus most easily blocked and travel only a few centimetres through air on average and can be blocked by a sheet or two of paper. Beta particles generally travel further though can be blocked by 5mm thick aluminium sheet. Gamma rays can travel long distances before they interact. About 10 cm of lead sheeting is needed to halve the intensity of the gamma radiation.

Having a charge of +2e and high mass, alpha particles readily ionise any atoms or molecules that they pass by.

Some of the properties of alpha, beta and gamma radiation are summarised in the following table:

Type of Radiation

α

β

γ

What is it?

Helium nucleus (2 protons, 2 neutrons)

electron

gamma photon

Charge

+2e

-1e

no charge

Energy (typical)

10 MeV

0.03 to 3 MeV

1 MeV

Speed (typical)

0.1c

range up to 0.9c

c

Penetration (typical)

stopped by 5 cm of air of 0.5 mm of paper

stopped by 0.5 cm of aluminium

intensity halved by 10 cm lead

Ionisation effect

high

medium

low

Deflected by magnetic (B) field?

Yes

Yes, opposite direction to α particle

No

Deflected by Electric (E) field?

Yes

Yes, opposite direction to α particle

No

Properties of Alpha, Beta and Gamma Radiation

Alpha, beta and gamma radiation also differ in how they are effected by magnetic and electric fields. Being positively charged, alpha particles are deflected by both magnetic and electric fields. Having a negative charge, beat particles are also deflected by both types of field but in the opposite direction to that of alpha particles. Gamma rays have no charge so are unaffected by magnetic and electric fields.

Positively-charged alpha particles are deflected in the direction of the E field lines. Negatively-charged beta particles are deflected in the opposite direction to that of the E field lines.

The direction of deflection for charged particles may be determined using Fleming's left hand rule. Positively-charged alpha particles will be deflected in the opposite direction to that of negatively-charged beta particles.